This application is based on Patent Application No. 2001-148008 filed May 17, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus for forming an image using a printing liquid (ink) and more particularly to an ink jet printing apparatus and an ink jet printing method which prevent an image quality degradation (smear) caused by a rubbing between print mediums during their discharge while minimizing a reduction in the efficiency of the printing operation. What is referred to here as xe2x80x9cinkxe2x80x9d includes not only those liquids used to attach desired colors to the print medium but also so-called transparent processing liquids applied to the print medium before or after the colors are applied.
In this specification, the printing or recording means having inks adhere to the print medium such as paper not only according to print data representing characters and images but also according to meaningless data, such as random data and solid print data.
2. Description of the Related Art
Printing apparatus such as printers, copying machines and facsimiles are intended to record images of dot patterns on a print medium, such as paper and a thin plastic sheet, according to image information.
The printing apparatus may be classified according to the printing system into an ink jet system, a wired dot system, a thermal system, a laser beam system, etc. Of these, the ink jet system (ink jet printing apparatus) ejects ink (printing liquid) droplets from nozzles of a print head and has them adhere to the print medium to form an image.
An increasing variety of printing apparatus has come to be used in recent years and there are growing demands on these printing apparatus for higher printing speed, higher resolution, higher print quality and reduced noise. Among the printing apparatus capable of meeting such requirements the ink jet printing apparatus can be cited.
In many of the ink jet printing apparatus, however, since inks used in the printing operation are aqueous liquids, they take time to be dried and fixed.
The problem about the fixing time tends to be recognized as an important problem of ink in accordance with improvement of the printing speed in recent years. That is, in a printing apparatus with a slow printing speed, there is a lot of time before the next page begins to be printed, and since fixing of the ink advances within the time, the fixing of the ink is not much of a problem. In recent printing apparatus capable of outputting five or more A4-size printed sheets in one minute, particularly those printing 10 sheets a minute, there is a possibility that a sheet of printed paper (or printed material) may be smeared by the ink on the previously printed paper when the previously printed paper and subsequently printed sheet contact each other. That is, when a printed sheet has an area printed with a somewhat high print duty, the next printed sheet is discharged before ink on the first printed sheet is completely dried. As a result, the second sheet may rub the incompletely dried portion of the first sheet. The phenomenon in which printed sheets are smeared with ink by the successively printed sheets rubbing each other is called a xe2x80x9csmearxe2x80x9d or xe2x80x9cdischarged paper-induced smear.xe2x80x9d
A conventional method commonly employed to solve the problem of smear during the paper discharging operation involves providing a fixing mechanism using a heater, or arranging in the printed sheet discharge mechanism a device for preventing a newly printed sheet from rubbing the previously printed sheet and then driving the printed sheet discharge mechanism when the printing is complete, thus successively stacking the printed sheets without smearing them.
The method that provides the mechanisms described above, however, is difficult to apply to small printers, particularly portable small printers. For example, the fixing mechanism using a heater can increase a power consumption and is required to have a heat insulation to prevent heat from adversely affecting circuits in the apparatus between other mechanism (for example, control circuit) in the apparatus and fixing mechanism. Considered in terms of running cost, apparatus size and apparatus cost, the fixing mechanism using a heater seems disadvantageous. That is, the use of the fixing mechanism will likely lead to an increase in the equipment size and cost. Particularly in a small portable printers using batteries, it is impractical to use the heater-based fixing mechanism with a large power consumption.
To prevent printed images from being rubbed during the printing operation, a system has also been proposed which has a printed sheet discharge mechanism that causes subsequently printed sheets to fall vertically onto the previously discharged sheets during the printed sheet stacking process. This mechanism, however, is complex and thus not suited for an ink jet printing apparatus that aims at a small size.
The present invention has been accomplished to solve the aforementioned problems and thereby provide an ink jet printing apparatus capable of preventing a discharged paper-induced smear with a simple construction.
To solve the problems described above, the present invention has the following construction.
According to one aspect, the present invention provides an ink jet printing apparatus which comprises: a print head for forming images by ejecting ink onto print mediums; a discharging means for discharging printed mediums successively to a predetermined discharge position; a duration determining means for determining, based on an amount of ink ejected per unit area onto a preceding print medium last discharged to the discharge position by the discharging means, a required time duration that needs to elapse before a subsequent print medium being discharged from the discharging means toward the discharge position is allowed to contact a predetermined area of the preceding print medium; a speed control means for controlling a printing speed on the subsequent print medium so that the subsequent print medium will not contact the predetermined area of the preceding print medium within the time duration determined by the duration determining means; a size detection means for detecting a size of the print mediums; and a modifying means for changing a timing of performing the printing speed control on the subsequent print medium according to a detection result produced by the size detection means.
According to another aspect of the invention, the ink jet printing apparatus may further comprise a discharged paper support means for supporting the discharged print medium, wherein the control means controls the printing speed on the subsequent print medium according to an operation state of the discharged paper support means and a detection result produced by the size detection means.
According to a further aspect, the invention provides an ink jet printing method which comprises the steps of: printing images by ejecting ink onto print mediums; discharging the printed mediums successively to a predetermined discharge position; determining, based on an amount of ink ejected per unit area onto a preceding print medium last discharged to the discharge position, a required time duration that needs to elapse before a subsequent print medium being discharged toward the discharge position is allowed to contact a predetermined area of the preceding print medium; controlling a printing speed on the subsequent print medium so that the subsequent print medium will not contact the predetermined area of the preceding print medium within the time duration determined by the duration determining step; detecting a size of the print mediums; and changing a timing of performing the printing speed control on the subsequent print medium according to a detection result produced by the size detection step.
In this invention with the construction described above, for printed portions where the ink fixing is complete, the printing operation can be done at high speed by continuing the printing operation without reducing the printing speed. For only those printed portions where the ink fixing is not complete and the discharged paper-induced smear is likely to occur, a delay printing can be activated before the front end of the subsequent print medium reaches the printed portions in question. Further, by setting the discharged paper-induced smear prevention control timing and the control on the paper discharge operation according to the size of the print medium, the smear control can be performed more efficiently, thereby minimizing a reduction in the printing speed while preventing the smear.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.